it would be the development of Vaccine
For example, Before vaccine was invented, many people in Europe died from small pox. Approximately, small pox alone took around 400,000 of European people each year. (to understand how bad it is, it's important to know that their total population at that time was so small compared to the current population)
The development of vaccine prevented European people during industrial revolution from infected from many type of illness that could easily harm them due to lack of knowledge to treat them.
